This invention relates generally to the development of electrostatic images, and more particularly concerns a development system having an electrostatic filtering system which allows a steady flow of air into a development housing and prevents toner emission therefrom.
The invention can be used in the art of electrophotographic printing. Generally, the process of electrophotographic printing includes sensitizing a photoconductive surface by charging it to a substantially uniform potential. The charge is selectively dissipated in accordance with a pattern of activating radiation corresponding to a desired image. The selective dissipation of the charge leaves a latent charge pattern that is developed by bringing a developer material into contact therewith. This process forms a toner powder image on the photoconductive surface which is subsequently transferred to a copy sheet. Finally, the powder image is heated to permanently affix it to the copy sheet in image configuration.
Two component and single component developer materials are commonly used. A typical two component developer material comprises magnetic carrier granules having toner particles adhering triboelectrically thereto. A single component developer material typically comprises toner particles having an electrostatic charge so that they will be attracted to, and adhere to, the latent image on the photoconductive surface.
There are various known development systems for bringing toner particles to a latent image on a photoconductive surface. Single component development systems use a donor roll for transporting charged toner to the development nip defined by the donor roll and the photoconductive surface. The toner is developed on the latent image recorded on the photoconductive surface by a combination of mechanical scavengeless development. A scavengeless development system uses a donor roll with a plurality of electrode wires closely spaced therefrom in the development zone. An AC voltage is applied to the wires detaching the toner from the donor roll and forming a toner powder cloud in the development zone. The electrostatic fields generated by the latent image attract toner from the toner cloud to develop the latent image. In another type of scavengeless system, a magnetic developer roll attracts developer from a reservoir. The developer includes carrier and toner. The toner is attracted from the carrier to a donor roll. The donor roll then carries the toner into proximity with the latent image.
One method of controlling toner emissions from developer housings in xerographic equipment is to relieve any positive pressure generated in the housing. Moving components such as the magnetic brush rolls and the mixing augers can pump air into the housing, causing slight positive pressures. These positive pressures can result in air flow out of the housing via low impedance leakage paths. This air escaping from the housing contains entrained toner and is a major potential source of dirt within the system. A common approach to relieving this pressure is through the use of a "sump sucker". In it's simplest form a sump sucker is a simple port into the air space above the developer material in the housing. This lowers the pressure in the housing below atmospheric pressure, therefore air flows into, rather than out of any low air impedance leakage paths within the housing. This toner laden air is drawn through a tube to a filter/waste sump assembly. A shortcoming of this system involves the waste toner removed from the system with this air flow. The amount of toner withdrawn from the system has a direct negative effect on total systems efficiency by increasing the total amount of waste toner, resulting in increased cost of ownership to the customer. Further, toner particle size distributions and additive concentrations can be affected if there is preferential air entrainment of some particles.
Further, the scavengeless development housings have decreased in size, thus, increasing magnetic roll speeds have been required to obtain adequate developability or donor reload in the case of HSD. Under these conditions toner emissions have increased and are considered a serious problem.